Wednesday, August 2, 2017

NYC is lowering its carbon emissions, mostly through
energy efficiency and conservation measures, and to a lesser degree, by adding
more solar and wind power. There’s an
entirely different avenue that so far, NYC hasn’t ventured into: carbon sequestration.

Carbon can be sequestered, or captured for long-term storage, before it can
escape into the atmosphere as carbon dioxide, which is what usually happens
when fossil fuels or organic solids are burned or decompose.

The most widely discussed methods of carbon sequestration
propose removing CO2 from exhaust gases at fossil fuel burning power stations,
and storing it in underground reservoirs, but these are unproven as well as
costly. Proven natural methods, such as
reforestation and raising carbon content of soil through regenerative
agriculture practices, don’t directly apply to urban environments.

However, it’s possible for NYC to sequester the carbon in sewage sludge, one of
its biggest municipal solid waste streams, by turning sludge into a type of
charcoal called biochar that can be reused or disposed of locally – potentially
saving many millions a year in dumping fees.

When organic solids are heated in the absence of oxygen, instead of catching on
fire, they turn into charcoal. Most
charcoal is burned as a fuel, but when made to meet certain standards, for a
wide range of specific applications, it’s referred to as biochar.

When added to soil, biochar improves agricultural productivity and water
retention. The structure of wood is preserved at the microscopic level, providing habitat for beneficial soil microbes and fungi.

This was common practice in South America’s Amazon region before the European arrival, as layers of unusually dark and fertile soils with high charcoal content attest.

Many forms of organic waste in NYC, such as paper, cardboard, yard waste and
leaves are already being recycled. Food
waste is increasingly being collected for composting. However,
the sewage waste biosolids left over at the end of the City’s water treatment
process are more difficult to dispose of.

NYC’s Department of Environmental Protection manages the City’s 14 wastewater
treatment plants, which together treat 1.3 billion gallons of wastewater
daily. Methane, the main constituent of
natural gas, is always produced in the anaerobic phase of sewage treatment, and
when waste decomposes in landfills. It’s
increasingly common for methane to be captured and burned at these locations to
heat water into steam, run a turbine, and produce electricity. Not only does this harvest a lucrative
resource, it also avoids serious harm:
methane escaping into the atmosphere has over 25 times the greenhouse gas
impact as carbon dioxide. There’s a
major initiative underway to collect methane at all 14 of the City’s plants and
to put all of it to beneficial use.

Whether or not gas is extracted from sewage, there’s still a lot of sludge at
the end of the process. NYC produces
1,400 tons per day of biosolids. Until it
was legally prohibited in 1988, the City was dumping it in the ocean.

As the City’s 2006 Solid
Waste Management Plan and its Biosolids
Management appendix explain, biosolids can be used as fertilizer when spread
on farms, parks, lawns, golf courses, and cemeteries, or it can be less
usefully dumped in landfills.

According to anecdotal reports from DEP staff, NYC biosolids are free of metal
contamination or biological risk.
They’re even safe to use as fertilizer for growing food for human
consumption. However, because of the
availability of other inexpensive fertilizers, the market for sewage biosolids may
be limited. NYC is paying contractors to
take virtually all of NYC’s biosolids into landfills. ​

At
$108 per wet ton, the City is now paying $151,200 per day in hauling fees, or
about $55,188,000 per year.

NYC may be better off turning its biosolids into biochar. Relatively low carbon organic solids like sewage
sludge or food waste must be mixed with woody biomass, high in both carbon and
lignin, before it can be used as a good feedstock for biochar.

Next questions for exploring biochar as
an option for NYC

Before the operating costs
of using this approach for NYC’s biosolids can be estimated, agency staff will have to consult with biochar scientists
and other experts and stakeholders to assess what feedstocks are needed to mix with
it, and what are available; the capacity of various biochar systems, their
purchase cost and costs of operation.

Even if turning some or all of the City’s biosolids to biochar would cost more than the $55 million annually in shipping costs paid now, other benefits (in addition to carbon sequestration) may outweigh the
extra cost.Potential feedstocks

The NYC Parks Department, the Department of Transportation, and Con Edison are
constantly pruning and removing trees within the City. How big is NYC’s supply
of waste wood chips, and where it is going now?

How much wood is in NYC’s construction and demolition waste stream, and how much
is being productively reused, or is going into landfills? Even if construction waste wood is chemically contaminated, charring it before
landfilling would permanently sequester much of its carbon content and reduce
its volume. Some contaminants can be rendered chemically inert or biologically unavailable
after the material they’re in is turned into biochar.

August updates: According to the NYC Department of Sanitation's 2013 Waste Characterization Study, the City produces 3.25 million tons of waste annually. Debris from construction and demolition has stayed the same as a percentage of aggregate discards between 2005 and 2013.In 2013, treated / contaminated wood made up 1.3% of that total, and 0.8% was untreated lumber, pallets and crates. That may be 42,000 tons of contaminated wood and 26,000 tons of untreated wood.

NYC will have to research which of the many manufacturers of biochar production
systems have a successful track record with municipal sewage biosolids, will be
best suited for a pilot project, and can potentially scale up to NYC-sized
volumes of sludge.

Combined heat and power
plants increase the energy efficiency of the fossil fuels they burn both by
generating electricity and creating steam or hot water at the same time. Can heat from existing in-city power
generation facilities also be used simultaneously to produce biochar? Does this
mean locating biochar ovens at power plants, and shipping waste for processing?
Or if biochar ovens are best located at sewage treatment plants, can captured
methane be burned for some or all of the heat they need?

Uses of biochar

In a project recognized by the European
Mayors Challenge and C40
Cities, the City of Stockholm has started turning plant waste into biochar,
and using it as a soil amendment for the city’s trees.

The Ithaka Institute
cites 55
uses for biochar. Some biochar processes can yield chemical byproducts that can be sold profitably to
industrial users.

If NYC can demonstrate a financially viable pathway for charring sewage
biosolids, the model could be applied globally, boosting NYC’s role as a leader
in climate response - and multiplying our carbon sequestration impacts.

Have some answers or suggestions? Please add your comment or contact me at danminer2345@gmail.com.